The present invention relates to audio systems and more specifically to an improved method and apparatus for providing reverberation.
A listener in a room hears a combination of direct sound emanating from the sound source and a series of reflections from the room surfaces, which occur at different times. The frequency response at the listener location contains many peaks and valleys due to comb filtering, as all of the reflections and direct sound add together vectorially. Early attempts at electronic reverberation used a loudspeaker and microphone in a non-absorbent room. Later, space was saved by replacing the room with a metal plate or springs. When electronic analog delay became available, a decaying train of pulses could be produced by recirculating the output back to the input at slightly reduced gain. The development of computation and analog-to-digital and digital-to-analog converters allowed the same decaying train of analog pulses to be produced in the digital domain.
Reverberation can be characterized by its impulse response. Mathematically convolving a music signal with this impulse response produces the reverberant signal. Therefore, development in reverberation has focused on obtaining a desirable impulse response. The latest method of producing electronic reverberation, now becoming popular, is to use sampling. Recording the impulse response of a concert hall and feeding it into a convolver makes a non-reverberant music source sound somewhat as though it was produced in that concert hall.
Because of the large dimensions of concert halls, sound absorption by the audience and surfaces, and the speed of sound at approximately 1090 feet per second, listeners in even the best concert halls hear the direct sound at least 15 milliseconds before the first significant surface reflections arrive. The extreme high frequency content of the reflected sound is greatly attenuated relative to the direct sound. At low frequencies, depending upon the seat location, the reverberant sound usually exceeds the loudness of the direct sound. Some people like singing in a ceramic tiled shower stall where the reflected sound arrives much sooner and has more high frequency content.
Electronic reverberation systems used in modern recordings have similar characteristics and provide more than 15 milliseconds of initial delay and attenuated high frequencies. The delay and lack of high frequency content in either the acoustic or artificial reverberation allows any noises or imperfections in the direct sound picked up by the microphones to be clearly heard.
Most people do not realize they are listening to a beat frequency that occurs among multiple instruments or voices sounding the same note. Depending upon frequency, phase, and harmonic differences, a listener may hear a shimmering effect or high frequency noise. In addition, bowed instruments produce mechanical noises, and wind instruments produce wind noises and occasionally annoying high harmonics. Percussion instruments have rattles, and voices can be raspy on certain notes. Close microphone techniques often exaggerate these imperfections.
Recording, transmission, and reproduction equipment may contribute their own imperfections or exaggerate those already present. For example, some recording engineers dislike the normal pulse code modulation (PCM) recording process due to irritating high frequency components they believe are not present in the live microphone signal. Lossy bit compression systems like MPEG-3 are also believed by some recording engineers to distort sound quality. Processes generally accepted by these same engineers are old-fashioned analog tape recording and new Direct Stream Digital (DSD) recording used in making Super Audio Compact Discs (SACD). Instead of 16-bit PCM at 44.1 kHz used for compact discs, DSD is 1 bit PCM at 2.7 MHz.
Regardless of the sources of high frequency imperfections, the cumulative result is that nearly all existing recordings contain moments when the high frequencies are irritating enough to cause the listener to turn the volume down below the point of maximum enjoyment for the rest of the program—and sometimes to just turn it off. High frequency irritants can be reduced by attenuating the high frequencies using an equalizer. However, attenuating the high frequencies enough causes unsatisfactory loss of high frequency detail.
It would therefore be desirable to have a system and method for reducing the imperfections, distortions and/or irritating effects exhibited by recorded material.